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In its elemental form, lithium is a soft, silvery-white metal. At room temperature, it is the lightest of all solid elements. Among the alkali metals, lithium has the highest melting and boiling points as well as the highest electrochemical potential, which enables very high energy and current densities in batteries. It also has the highest specific heat capacity of solids and a low density (0.53 g/cm³). This makes lithium particularly well suited for long-term use in small and lightweight batteries.
Lithium represents a highly versatile metal. It is indispensable in glass and ceramic processing, but also in lubricants and a variety of special salts. Lithium is also used in pharmaceuticals. For some years now, the use of lithium has been increasing, especially in rechargeable batteries, i.e. lithium-ion batteries, both in consumer electronics (tablets, smartphones) and in electric vehicles.
Currently, global end-use markets are estimated as follows: batteries (74%); ceramics and glass (14%); lubricating greases (3%); continuous casting molding powders (2%); polymer manufacturing (2%); air preparation (1%); and air treatment (1%); and other uses (4%).
Pure lithium is very reactive in air. It is therefore produced and processed in the form of various lithium salts. Lithium carbonate and lithium hydroxide dominate here, but also lithium chloride. Due to the different density of these compounds, the term "lithium carbonate equivalent" (LCE) was introduced for better comparability.
Since 2019, the use of lithium hydroxide is preferred for solid-state synthesis of cathode materials, as it allows for rapid and complete synthesis at lower temperatures. This improves the performance and lifetime of a lithium ion battery. For these applications, the highest purity of lithium salts of at least 99.5% (Battery Grade) is required.
For lithium extraction from hot deep water, various options are available, which are summarized under the generic term "direct lithium extraction processes (DLE)". A promising method for the direct extraction of lithium from thermal waters is sorption or ion exchange processes. The sorption takes place on inorganic sorbents, such as manganese oxides. These have a high selectivity for lithium ions, which is advantageous due to the high salt load of the thermal waters. Therefore, the UnLimited project team decided to use this process.
Other processes being investigated in connection with hot deep waters include liquid-liquid extraction, coprecipitation on aluminum compounds, and ion exchange resins and membrane processes.
Many deep geothermal waters in Germany are highly mineralized. Total solution contents of over 100 g/L are known for the Upper Rhine Graben. This corresponds to about four times that of seawater. While sodium and chloride dominate, lithium only exists in traces with approx. 0.1 % of the total solution content in the thermal water. The extraction of lithium therefore has very little effect on the chemistry of the thermal water.
The high selectivity of manganese oxide sorbents for lithium is due to the special crystal structure. The sorbent has small tunnels, similar to a sponge, in which only lithium can sorb. Other elements dissolved in the deep water are therefore not sorbed at all or only in small amounts. This expected behavior has already been proven in our own laboratory tests. The practical confirmation at a geothermal plant in operation is still pending and is an important point of investigation in the UnLimited project. For this purpose, an elaborate hydrochemical monitoring system is being set up at the Bruchsal site and the lithium selectivity of the sorption process is being monitored and evaluated in practical use.
The adsorption capacity expresses how many milligrams of lithium per gram of sorbent can be absorbed from the geothermal water and depends on several factors. It is mainly determined by the initial concentration of lithium in the thermal water and the ratio of the amount of sorbent used to the volume of thermal water. Experiments already carried out within UnLimited reveal a sorption capacity of up to 18 mg/g for manganese oxides.
Due to their different modes of operation, the direct extraction processes (DLE) can be harmonized with the existing power plant operation to varying degrees. While lithium extraction by means of coprecipitation, for example, complicates the plant technology due to several process steps with large time and space requirements, sorption processes fit into the existing infrastructure of a geothermal plant in a much simpler and also space-saving way.
In the tests for UnLimited, a partial flow of the extracted thermal water is essentially passed through an extraction unit via a bypass. The actual adsorption takes very little time. The lithium-depleted thermal water is completely returned to the subsurface without emissions. The actual power plant operation can thus continue without restrictions.
For the annual production of 800 tons, two wells are required at a production rate of 30 liters per second. These are available at the Bruchsal site. It is also important to note at this point that the various lithium salts and, above all, elemental lithium have very different densities. For example, one kilogram of elemental lithium corresponds to about 5.3 kilograms of lithium carbonate. Conversely, this means that for the extraction of one kilogram of lithium carbonate, only about 200 g of lithium need to be extracted from the thermal water.
Resources are deposits that cannot be operated at a given time from an economic and technical point of view. Usually, resources are divided into suspected, indicated and measured resources depending on the geological state of knowledge. As a result of advancing exploration, the amount of identified lithium resources worldwide has increased significantly and currently stands at approximately 89 million tons (U.S. Geological Survey). The 10 countries with the largest lithium resources worldwide are Bolivia (21 million tons), Argentina (19.3 million tons), Chile (9.6 million tons), Australia (6.4 million tons), U.S. (7.9 million tons), China (5.1 million tons), Congo (3 million tons), Canada (2.9 million tons), Germany (2.7 million tons) and Mexico (1.7 million tons).
Reserves, on the other hand, are the quantity of an already developed raw material that can be economically extracted with the currently available technical possibilities. It is often divided into the probable reserve and the assured reserve. The reserve is generally calculated by a mining company according to certain standards (e.g. JORC) and confirmed by an independent expert. Due to its dependence on the economic and technical situation, the reserve represents a variable quantity that only allows limited statements on the availability of raw materials. In total, lithium reserves worldwide amount to about 22 million tons (U.S. Geological Survey). Accordingly, the countries with the largest lithium reserves are Chile (9.2 million tons), Australia (5.7 million tons), Argentina (2.2 million tons) and China (1.5 million tons).
The rough estimation of lithium resources in the Upper Rhine Graben and North German Basin is the task of UnLimited. As an example for Germany, detailed investigations are carried out at selected test sites. The focus is on the dissolved concentrations of lithium in the thermal water and their changes over time as well as the determination of the origin of the lithium. The aim is to determine both the size of the deposits and possible recovery rates as well as the sustainability of the exploitation.
Lithium is not a rare element, but it mostly not concentrated in deposits high enough to make extraction worthwhile. Currently, the most economically important sources of the element worldwide are solid rock deposits and salt lakes, known as brine deposits. These two share the global supply of lithium almost equally. Besides salt lakes and solid rocks, river water (about 3 µg/L) and seawater (about 180 µg/L) also contain lithium. The hot deep waters in the Upper Rhine Graben contain lithium on the order of about 150 to 200 milligrams of lithium per liter, 1000 times higher than seawater. There are several approaches to explain the origin of these lithium contents, such as water-rock interactions and hydrothermal processes. The origin of the lithium will be further investigated in the UnLimited project.
The deep waters in the Upper Rhine area show lithium concentrations of up to 200 mg/L. At a production rate of 50 L/s, 8000 operating hours and a lithium recovery rate of 70%, lithium carbonate in the order of circa 1,000 tons per year could be produced in the Upper Rhine Graben. The original origin of lithium in deep waters is largely unknown so far, but it is an important parameter for the sustainable management of the resource. In order to clarify the question of sustainability, a tracer experiment is being carried out in the UnLimited joint project.
Lithium is a trace element, i.e. it is present in a rock with a content of less than 0.1% by weight. Therefore, the formation of cavities as known from classical mining cannot occur. It can rather be assumed that due to water-rock interactions lithium has already been dissolved from lithium-bearing minerals by contact with hot, very old deep waters and has been enriched in the deep waters over time.
Geothermal plants in Germany all operate within a closed cycle. This means that neither gases nor liquids are released into the environment. Sorption is an environmentally friendly process, and the sorbent applied can be used over several extraction cycles. The extent to which naturally occurring radionuclides, which occur in rocks and also in ground and deep waters, are adsorbed during lithium extraction is being investigated in detail by a hydrochemical monitoring system in UnLimited. The results are incorporated into a life cycle assessment, which is used, among other things, to evaluate the environmental compatibility of the sorbents used.
Initial investigations have shown that sorption processes in particular are characterized by low energy consumption compared to other lithium extraction processes. Exactly how high this amount is, is to be determined in the UnLimited project.
The sorbents used are washed with a desorbent after loading with lithium and are then directly ready for use again for the next loading cycle. In laboratory tests, the manganese oxide sorbent showed high stability and thus reusability for many cycles. This lab result will be further investigated in the UnLimited project.
The amount of lithium extracted annually at the geothermal plant in Bruchsal alone during 8,000 operating hours is sufficient for the production of about 20,000 batteries for use in electric cars (assumptions: 60 kWh battery capacity, lithium extraction rate: 70%). At a production rate of 30 L/s, approximately 800 tons of lithium chloride are extracted per operating year.
Whether and how a lithium extraction from hot deep waters in the Upper Rhine region can prove itself in competition with lithium from South America or Australia, for example, is still unclear at present and therefore an important point of investigation in the UnLimited project. Recent publications by government research institutions in the USA cite operating costs of about €4,000 per ton of lithium salt at a geothermal site in Salton Sea (Utah). This amount is not dissimilar to the cost of lithium extraction in traditional deposits in use today. To what extent the cost estimates can be compared with plants in the Upper Rhine region, for example, remains to be seen from our own investigations. What is important, however, is that long transport distances and the associated costs and environmental impacts can be avoided for the production of lithium batteries in Germany with lithium from the Upper Rhine.
There are many factors that influence the economic viability of lithium extraction from hot deep waters. The most important are the plant costs and the energy consumption, but also the lithium content of the thermal water, as well as the extraction rates and much more. Therefore, the economic efficiency of geothermal lithium extraction cannot be measured by a single threshold value.
The lifetime of a geothermal plant is usually assumed to be about 30 years or longer. In Larderello, Tuscany, electricity production from geothermal energy can already trace back more than 100 years. Whether lithium can be extracted in addition to energy over a comparable period of time has yet to be proven. This question is a major focus of the UnLimited project.
Plans to extract lithium from seawater are not entirely new. Seawater contains lithium in low concentrations of 0.2 mg/L on average. This value is lower by a factor of 1000 than the lithium contents of the hot deep waters at the Upper Rhine. Consequently, in order to extract lithium from seawater on a larger scale, very large quantities of seawater and thus a high energy input are necessary. Whether and to what extent lithium can be extracted from seawater under economic conditions and with a low environmental footprint is therefore still uncertain at present.
In principle, the direct extraction processes can be transferred to other sites with geothermal use. Also, it is already being investigated whether direct lithium extraction can be used at South American salt lakes, i.e. a classic deposit. Whether these efforts will be successful, however, depends strongly on the respective site conditions.
A complete value network that maps battery production for an e-car on a large industrial scale, for example, is only just being established in Europe. European companies are therefore heavily dependent on battery cell imports. Testing new technologies for lithium extraction from deep waters can contribute significantly to the completion of a regional value chain. Local extraction of the raw material lithium in Germany is also of particular importance with regard to avoiding long transport routes and dependencies. A successful implementation of lithium extraction from geothermal waters would also contribute to making Germany more independent from global raw material markets, as well as buffering global price fluctuations and supply difficulties. At the same time, co-production of lithium can lead to positive economic effects for the geothermal industry.
The UnLimited research project deals with lithium extraction from deep geothermal water in Germany. The research and investigations carried out particularly cover the region of the Upper Rhine Graben and the North German Basin, as the highest lithium contents in thermal waters within Germany have been documented there. The overall objective of the project is the development and testing of a suitable process that should enable lithium production from the extracted deep waters concomitant to their geothermal use. This will be accompanied by an evaluation of the sustainability of geothermal lithium production, as well as the related origin and mobility of lithium in geothermal reservoirs.
UnLimited represents a joint project led by Energie Baden-Württemberg AG (EnBW) and four other project partners, including the project development company Bestec, the consulting firm Hydrosion, the Karlsruhe Institute of Technology (KIT), and the Georg-August University of Göttingen.
The research project is funded by the German Federal Ministry of Economics and Climate Protection as a result of a resolution of the German Bundestag. The focus lies on the possibility of using a domestic resource as a funding policy purpose.